Wash and hull the strawberries, then crush them with a potato masher. Leave some small pieces for a homemade fruit texture rather than puréeing the berries completely.

Wash the rhubarb thoroughly, trim away both ends, and chop the stalks finely. Never use rhubarb leaves, because they are not suitable for eating.

Measure exactly 4 cups of prepared rhubarb and 4 cups of prepared strawberries into a large, deep stockpot. Add the bottled lemon juice and powdered fruit pectin, stirring until evenly combined.

Set the pot over medium-high to high heat and bring the fruit mixture to a full rolling boil, stirring frequently. The boil should continue even while you stir, and the deep pot helps contain the vigorous foam.

Add all of the sugar at once and stir constantly until it dissolves. Return the mixture to a full rolling boil, watching carefully because strawberry-rhubarb jam can foam vigorously.

Boil the jam hard for exactly 1 minute, continuing to stir. Remove the pot from the heat immediately afterward and skim away foam if needed.

Ladle the hot jam into hot, sterilized half-pint jars, leaving about 1/4 inch of headspace. Wipe each rim clean, apply the two-piece lids, and process the jars in a boiling-water canner for 10 minutes, adjusting the time for altitude.

Remove the jars from the canner and let them cool undisturbed for 12 to 24 hours. Check the seals once cool, and refrigerate any jar that has not sealed properly.